The present invention relates to a method for chemical vapor deposition and particularly relates to a method and an apparatus for producing a thin film by vapor deposition on a surface of a high technology product such as semiconductor, liquid crystal, precision instrument in a process for producing the product.
In a process for producing a semiconductor, particles are generated in a step of producing a thin film onto a surface of a substrate by CVD. The particles refer to particulate materials which have non-uniform diameters, and which are made by interacting components for thin films with accompanying impurities in a reaction chamber of the CVD apparatus and aggregating the reactants. The particles are contaminants to the thin film and a main factor to reduce a yield of the product. Recently, as semiconductor circuits are becoming finer and finer, a size of particulates that may act as contaminants is getting smaller and smaller. Pattern sizes of 64M DRAM, 256M DRAM, 1G DRAM and 1T DRAM are about 0.35 xcexcm, about 0.25 xcexcm, about 0.18 xcexcm, about 0.09 xcexcm, and about 0.03 xcexcm, respectively. It is considered that particulates having sizes not less than one tenth of the pattern size should be removed as contaminants. Highly reactive materials are frequently used as ingredient gases, and such materials are prone to generate particles. Therefore, a CVD apparatus is sometimes called xe2x80x9can apparatus for generating trash.xe2x80x9d
Conventionally, an amount of ingredient gases and temperatures are adjusted to decrease a rate of producing a thin film, thereby preventing particles from generating. However, the decreased rate of producing a thin film reduces an output of semiconductor products. Alternatively, since particles fall by gravitational force in the reaction chamber, a surface of the wafer which a thin film is formed onto may be arranged to face down. Alternatively, thermal migration or temperature fluctuations are generated in the reaction chamber so as to decrease an amount of particles accumulated onto a wafer. Although these processes are effective to decrease an amount of particles accumulated to a certain extent, these process may not produce a sufficient result.
A conventional CVD apparatus transfers a component for a thin film by molecular diffusion or thermal diffusion to a substrate and deposit thereon. However, the conventional CVD apparatus has a problem that a rate of producing a thin film is slow and that an amount of the deposition is small.
An example for generating particles is described in a vapor deposition step using a plasma CVD apparatus. FIG. 13 is a schematic view of a plasma CVD apparatus. The apparatus is used to produce an SiO2 film onto a surface of a wafer 6 using an SiH4 gas as reactants. A pressure in a reaction chamber 21 is reduced by a vacuum pump 22. Subsequently, reactive gases namely an SiH4 gas serving as an ingredient gas and an N2O gas serving as an oxidative gas are mixed together by a mass flow controller 23, and then introduced into the reaction chamber 21. The pressure in reaction chamber 21 is kept constant by a pressure adjusting valve 24. The wafer 6 is heated by a heater 26, which is provided inside a support 13 for a wafer, to a constant temperature.
A high frequency voltage is applied to an upper electrode 29 and the support 13 for a wafer by an electric source 28 to generate plasma in a space therebetween so that SiH4 radicals, SiH4+ ions, N2O radicals and O2xe2x88x92 ions are present in the space. When these radicals and ions reach to a surface of the wafer 6, upon receiving thermal energy, an SiO2 film is formed.
These radicals and ions, however, have high activity and interact with reaction products and accompanying impurities to give particles. Thermal energy reaches to a sidewall of the reaction chamber by thermal radiation; and therefore an SiO2 film is formed onto the sidewall also. Furthermore, SiH3xe2x88x92 ions, SiH22+ ions, NO molecules and so on secondarily generate, and therefore there is a problem that quality of the film may not be uniform.
In view of the foregoing, it is an object of the present invention to provide a method and an apparatus for chemical vapor deposition which enable to produce a uniform film by preventing the generation of particles in the step of producing a thin film onto a surface of a substrate and which enable to form the thin film at a higher rate.
According to the present invention, there is provided a method for producing a thin film, which comprises the steps of: introducing a reactive gas into a reaction chamber wherein a substrate is supported in the reaction chamber; combining charged particles with a component of the reactive gas for ionizing the component; and electrostatically depositing the ionized component onto the substrate in an electric field. Preferably, the charged particles are photoelectrons or positive or negative ions produced by corona discharge, surface discharge or pulse discharge. Namely, in the present invention, ionization of a component constituting the reactive gas is conducted by using photoelectrons or ions, which may be referred to as charged particles. The reactive gas may be solely an ingredient gas containing a component for a thin film or a mixture of the ingredient gas and an oxidizing or reducing gas.
An apparatus for chemical vapor deposition, which carries out the aforementioned process, comprises: a reaction chamber including a support for a substrate; means for introducing a reactive gas containing a component for a thin film into the reaction chamber; means for discharging the reactive gas that passes through the reaction chamber; ionizing means for ionizing a component constituting the reactive gas; and means for forming an electric field in the reaction chamber so that the ionized component for a thin film is electrostatically deposited onto the substrate. Preferably, the ionizing means is means for producing photoelectrons or means for producing ions by electric discharge. Preferably, the ionizing means is provided in a passage for introducing the reactive gas into the reaction chamber.